#!/bin/bash
#
# Raspberry Pi Fan Control Script
#
# This script toggles a GPIO output pin when a system temperature limit is exceed to control a cooling fan.
#
# usage:
#
#   $ ./fancontrol.sh
#
#   $ # show temp readings and fan state
#   $ DEBUG=1 ./fancontrol.sh
#
#   $ # set monitor freq to 2s and temp threshold to 55 degrees
#   $ FREQ=2 THRESHOLD=55000 ./fancontrol.sh
#

# The GPIO Pin number to use for switching the fan. Default = 18
PIN=${PIN:-18}

# Temp monitoring frequency (in seconds). Default = 5
FREQ=${FREQ:-5}

# Temp threshold. Fan will turn on when the temperature exceeds this value (in millidegrees). Default = 70000
THRESHOLD=${THRESHOLD:-70000}

function init {
        echo $PIN > /sys/class/gpio/export 2> /dev/null
        if [ $? != 0 ]; then
                >&2 echo "$(date): fan control failed to start"
                exit 1
        fi
        sleep 0.25
        echo "out" > "/sys/class/gpio/gpio$PIN/direction"
        echo "$(date): fan control started ($THRESHOLD/$FREQ)"
        [ ! -f $DEBUG ] && echo "$(date): debug: exported GPIO Pin $PIN -> /sys/class/gpio/gpio$PIN"
}

function cleanup {
        echo "$PIN" > /sys/class/gpio/unexport 2> /dev/null
        [ ! -f $DEBUG ] && echo "$(date): debug: unexported GPIO Pin $PIN"
        echo "$(date): fan control stopped"
}

function fan_on {
        [ $FAN_STATE = 0 ] && echo "1" > "/sys/class/gpio/gpio$PIN/value"
        FAN_STATE=1
}

function fan_off {
        [ $FAN_STATE = 1 ] && echo "0" > "/sys/class/gpio/gpio$PIN/value"
        FAN_STATE=0
}

FAN_STATE=0
CURR_TEMP=0
init
trap cleanup EXIT
set -e
while true
do
        CURR_TEMP=$(cat /sys/class/thermal/thermal_zone0/temp)
        [ ! -f $DEBUG ] && echo "$(date): debug: Temp=$CURR_TEMP, Fan=$FAN_STATE"
        if [ $CURR_TEMP -gt $THRESHOLD ]; then
                fan_on
        else
                fan_off
        fi
        sleep $FREQ
done